The present invention relates to heat pipes and in particular to heat pipes formed by the extrusion of a thermally conductive material through a die resulting in an axially grooved pipe of uniform cross section.
Conventional heat pipes operate to transfer heat from a heat source, where heat energy is produced or collected, to a heat sink, where the heat is stored or used. The usual configuration is a closed chamber containing a working fluid which absorbs heat by evaporation and releases heat by condensation in a continuous cycle. Thus the heat pipe may be characterized as having three sections: (1) an evaporator, located in the heat source region; (2) a condenser in the heat sink region; and (3) a transport section through which vaporized and liquid working fluid flow from the evaporator to the condenser and back.
A persistent problem in the design of heat pipes has been the provision of satisfactory means for moving the liquid working fluid from the condenser to the evaporator. Generally such means comprise capillary flow channels in or along the walls of the transport section, while the central region of the pipe's cross section is reserved for vapor flow in the opposite direction.
Several heat pipe designs incorporate a separate screen or mesh wicking element to supply capillary channels; examples are U.S. Pat. No. 3,971,435 and 4,116,266, issued to Peck and Sawata et al, respectively. While improving axial flow of the working fluid in the transport section, however, separate wicking elements invariably reduce heat transfer efficiency in the evaporator and condenser sections. Furthermore, the wicking elements must be produced by additional plating or forming techniques, and the performance of the heat pipe may suffer if there is any deformation of the wicking element during wick assembly or as a result of thermal stress.
U.S. Pat. No. 3,402,767, issued to Bohdansky et al, discloses a heat pipe having a plurality of narrow axial grooves which by themselves serve as capillary channels to transport the condensed working fluid, avoiding the problems of a separate wicking element. Again, however, the rectangular groove profile of Bohdansky is inefficient with respect to both the channelling of the condensed working fluid into the capillary grooves in the condenser area and in the transfer of heat through the working fluid, especially when the fluid has, as is typical, a low thermal conductivity.
The problem of optimizing the groove profiles for the evaporator, condenser and transport sections of an axially grooved heat pipe is addressed by U.S. Pat. Nos. 3,528,494 and 3,537,514, both issued to Levendahl. In essence, Levendahl proposes a distinct profile for each section of the heat pipe. An inner wall similar to that of Peck is suggested for use in the transport section only, so as not to impair the evaporator and condenser efficiencies. Levendahl further recognizes the effect on evaporator/condenser efficiency of varying the radius of curvature of the axial groove entrances. However, the Levendahl configuration requires that the individual evaporator, condenser and transport sections be formed separately and subsequently joined together, thus introducing considerable production costs.
In fact, production costs present a major obstacle in the design of an optimum groove profile. U.S. Pat. No. 3,566,651, issued to Tlaker, discloses a method for forming tubular parts by material displacement of the interior walls of a blank workpiece or pipe. Such deformation is accomplished by feeding the blank tube past a tapered mandrel and appropriately shaped die positioned within the tube. Another well known method for forming tubular parts is extrusion, which entails the feeding of the material from which the tube is formed past a die suspended by spider legs. The material is fed past the die in a semi-molten state, and fuses together as it passes the spider arms.
Both the Tlaker material displacement and the extrusion methods, while desirable from a low production cost standpoint, are limited with respect to the complexity of the axial groove configurations which may be formed thereby.
It would be of considerable advantage, therefore, to provide a heat pipe having optimum capillary flow means and heat transfer characteristics, yet which may be produced in meaningful quantities by economical techniques.